Retroreflective materials are characterized by the ability to redirect light incident on the material back toward the originating light source. This property has led to the widespread use of retroreflective sheeting for a variety of traffic and personal safety uses. Retroreflective sheeting is commonly employed in a variety of articles, for example, road signs, barricades, license plates, pavement markers and marking tape, as well as retroreflective tapes for vehicles and clothing.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as “beaded” sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Due to the symmetrical geometry of beaded retroreflectors, microsphere based sheeting exhibits the same total light return regardless of orientation, i.e. when rotated about an axis normal to the surface of the sheeting. Thus, such microsphere-based sheeting has a relatively low sensitivity to the orientation at which the sheeting is placed on a surface. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Cube corner retroreflective sheeting typically comprises a thin transparent layer having a substantially planar front surface and a rear structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element.
Cube corner retroreflective sheeting is commonly produced by first manufacturing a master mold that has a structured surface, such structured surface corresponding either to the desired cube corner element geometry in the finished sheeting or to a negative (inverted) copy thereof, depending upon whether the finished sheeting is to have cube corner pyramids or cube corner cavities (or both). The mold is then replicated using any suitable technique such as conventional nickel electroforming to produce tooling for forming cube corner retroreflective sheeting by processes such as embossing, extruding, or cast-and-curing. U.S. Pat. No. 5,156,863 (Pricone et al.) provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retroreflective sheeting. Known methods for manufacturing the master mold include pin-bundling techniques, direct machining techniques, and techniques that employ laminae.
In pin bundling techniques, a plurality of pins, each having a geometric shape such as a cube corner element on one end, are assembled together to form a master mold. U.S. Pat. No. 1,591,572 (Stimson) and U.S. Pat. No. 3,926,402 (Heenan) provide illustrative examples. Pin bundling offers the ability to manufacture a wide variety of cube corner geometries in a single mold, because each pin is individually machined. However, such techniques are impractical for making small cube corner elements (e.g. those having a cube height less than about 1 millimeter) because of the large number of pins and the diminishing size thereof required to be precisely machined and then arranged in a bundle to form the mold.
In direct machining techniques, a series of grooves are formed in the surface of a planar substrate (e.g. metal plate) to form a master mold comprising truncated cube corner elements. In one well known technique, three sets of parallel grooves intersect each other at 60 degree included angles to form an array of cube corner elements, each having an equilateral base triangle (see U.S. Pat. No. 3,712,706 (Stamm)). In another technique, two sets of grooves intersect each other at an angle greater than 60 degrees and a third set of grooves intersects each of the other two sets at an angle less than 60 degrees to form an array of canted cube corner element matched pairs (see U.S. Pat. No. 4,588,258 (Hoopman)). In direct machining, a large number of individual faces are typically formed along the same groove formed by continuous motion of a cutting tool. Thus, such individual faces maintain their alignment throughout the mold fabrication procedure. For this reason, direct machining techniques offer the ability to accurately machine very small cube corner elements. A drawback to direct machining techniques, however, has been reduced design flexibility in the types of cube corner geometries that can be produced, which in turn affects the total light return.
In techniques that employ laminae, a plurality of thin sheets (i.e. plates) referred to as laminae having geometric shapes formed on one longitudinal edge, are assembled to form a master mold. Techniques that employ laminae are generally less labor intensive than pin bundling techniques because fewer parts are separately machined. For example, one lamina can typically have about 400-1000 individual cube corner elements, in comparison to each pin having only a single cube corner element. However, techniques employing laminae have less design flexibility in comparison to that achievable by pin bundling. Illustrative examples of techniques that employ laminae can be found in EP 0 844 056 A1 (Mimura et al.); U.S. Pat. No. 6,015,214 (Heenan et al.); U.S. Pat. No. 5,981,032 (Smith); and U.S. Pat. No. 6,257,860 (Luttrell).
The base edges of adjacent cube corner elements of truncated cube corner arrays are typically coplanar. Other cube corner element structures, described as “full cubes” or “preferred geometry (PG) cube corner elements”, typically comprise at least two non-dihedral edges that are not coplanar. Such structures typically exhibit a higher total light return in comparison to truncated cube corner elements. Certain PG cube corner elements may be fabricated via direct machining of a sequence of substrates, as described in WO 00/60385. However, it is difficult to maintain geometric accuracy with this multi-step fabrication process. Design constraints may also be evident in the resulting PG cube corner elements and/or arrangement of elements. By contrast, pin bundling and techniques that employ laminae allow for the formation of a variety of shapes and arrangements of PG cube corner elements. Unlike pin bundling, however, techniques that employ laminae also advantageously provide the ability to form relatively smaller PG cube corner elements.
The symmetry axis of a cube corner is a vector that trisects the structure, forming an equal angle with all three cube faces. In the aforementioned truncated cubes of Stamm, the symmetry axis is normal to the equilateral base triangle and the cubes are considered to have no cant or tilt. The nomenclature “forward canting” or “positive canting” has been used in the cube corner arts to describe truncated cube corner elements canted in a manner that increases only one base triangle included angle relative to 60°. Conversely, the nomenclature “backward canting” or “negative canting” has been used in the cube corner arts to describe cube corner elements canted in a manner that increases two of the included angles of the base triangle relative to 60°. See U.S. Pat. No. 5,565,151 (Nilsen) and U.S. Pat. No. 4,588,258 (Hoopman). Canting of PG cube corner elements is described in U.S. Pat. No. 6,015,214 (Heenan et al.).
Canting cube corner elements either backward or forward enhances entrance angularity. Full cube corner elements have a higher total light return than truncated cube corner elements for a given amount of cant, but the full cubes lose total light return more rapidly at higher entrance angles. One benefit of full cube corner elements is higher total light return at low entrance angles, without substantial loss in performance at higher entrance angles.
A common method for improving the uniformity of total light return (TLR) with respect to orientation is tiling, i.e. placing a multiplicity of small tooling sections in more than one orientation in the final production, as described for example in U.S. Pat. No. 4,243,618 (Van Arnam), U.S. Pat. Nos. 4,202,600; and 5,936,770 (Nestegard et al.). Tiling can be visually objectionable. Further, tiling increases the number of manufacturing steps in making the tooling employed for manufacture of the sheeting.
In addition to being concerned with the TLR, the performance of retroreflective sheeting also relates to the observation angularity or divergence profile of the sheeting. This pertains to the spread of the retroreflected light relative to the source, i.e. typically, vehicle headlights. The spread of retroreflected light from cube corners is dominated by effects including diffraction, polarization, and non-orthogonality. For this purpose, it is common to introduce angle errors such as described in Table 1 of column 5 of U.S. Pat. No. 5,138,488 (Szczech).
Similarly, Example 1 of EP 0 844 056 A1 (Mimura) describes a fly cutting process in which the bottom angles of V-shaped grooves formed with a diamond cutting tool were slightly varied in regular order, three types of symmetrical V-shaped grooves having depths of 70.6 μm, 70.7 μm and 70.9 μm were successively and repeatedly cut at a repeating pitch of 141.4 μm in a direction perpendicular to the major surfaces of the sheets. Thus, a series of successive roof-shaped projections having three different vertical angles of 89.9°, 90.0°, and 91.0° in a repeating pattern were formed on one edge of the sheets.
Although the art describes a variety of retroreflective designs and their measured or calculated retroreflective performance; industry would find advantage in retroreflective sheeting having new cube corner optical designs and methods of manufacturing, particularly those features that contribute to improved performance and/or improved manufacturing efficiencies.